This invention relates to the determination of an identifying bit sequence in a radio frequency (RF) waveform and, more particularly, to the asynchronous detection and determination of such an identifying bit sequence without digital demodulation.
The Narrowband Advanced Mobile Phone Service (N-AMPS) and the Global System for Mobile-telecommunication (GSM) cellular radio standards use identifying bit sequences to identify signals as belonging to a particular cell. The base station for a cell determines the identifying bit sequence of a received signal to verify that the received signal has the expected bit sequence for that cell. If the received signal does not contain the expected bit sequence, the signal is rejected as being an interfering signal belonging to a neighboring cell. Although the term bit may suggest a strictly binary digit, this term is to be understood as also including a multi-level symbol used by higher-order modulation.
Currently, an identifying bit sequence is determined by demodulating an RF waveform containing a digital bit sequence into a sequence of bits, which is then compared to an expected bit sequence. Such a digital demodulation requires that the samples entering the digital demodulator be synchronized with the bit rate. To acquire synchronization, the demodulator must monitor the signal for a period of time. In a weak and fading environment, too much time may elapse before a reliable and accurate synchronization is achieved.
The information provided by an identifying bit sequence is most important in the adverse environments that make synchronization difficult and unreliable; decisions that rely on this information need to be made quickly, before already poor-quality connections become completely unusable. What is needed is a way to determine an identifying bit sequence without having to perform digital demodulation, and so avoiding the difficulties in having to synchronize the demodulator with the bit rate of the identifying bit sequence.